Coronary atherosclerotic heart disease, which is often referred to as a “coronary heart disease”, is a kind of heart disease caused by myocardial ischemia, anoxia or necrosis due to vascular stenosis or obstruction resulting from coronary artery atherosclerotic pathological changes. Cardiac stents are widely used as implants for the treatment of such cardiovascular diseases. Generally, a cardiac stent is delivered to the diseased area through a delivery system, and then the stent is expanded to reconstruct the blood vessels to treat the “coronary heart diseases” effectively.
The existing stents for treating coronary heart diseases are mainly divided into the following two categories: non-absorbable stents and absorbable stents. The framework of the conventional non-absorbable stents is usually made of stainless steel, nickel-titanium alloy or cobalt-chromium alloy. After being implanted into the body, the visibility of a metal-based stent is primarily dependent on the framework material and the stent thickness, and the stent thickness refers to the wall thickness of the tubular structure of the stent. If the thickness of a metal-based stent is greater than 80 μm, its image can be displayed clearly on the medical imaging equipment and digital subtraction angiography (DSA) equipment, so that the position and configuration of the stent can be easily identified, providing high visibility. For a metal-based stent, the smaller the stent thickness, the better the stent is apposed to the vascular wall, as such the strut would have less shear disturbance on the blood flow inside the blood vessel, which is more helpful for preventing thrombosis formation. Accordingly, a metal-based stent with a relatively thin thickness is preferred in medical practice. However, when the thickness of a metal-based stent is less than 80 μm, its image displayed on the DSA equipment may be unclear, so that the position and configuration of the stent is not easy to identify, leading to the need for improvements in visibility.
Absorbable stents are generally made of an absorbable metal (such as magnesium and iron) and polymer (such as polylactic acid, polycaprolactone or a copolymer thereof). As the polymer material itself has a very small density and relatively low visibility, an intravascular stent that is made of the polymer with wall thickness from 120 μm to 180 μm, i.e., a polymer stent, is almost invisible with the aid of medical imaging equipment and digital subtraction angiography equipment, so that physicians are unable to accurately locate the stent during surgical operations. Therefore, the polymer stent requires an additional radiopaque structure which can be identified by physicians under DSA. By identifying the position of the radiopaque structure, a physician can determine the position or configuration of the entire stent. Namely, the problem of the low visibility of the stent framework can be addressed by using a radiopaque structure with high visibility. Therefore, the visibility of the entire stent can be improved.
For a metal-based absorbable stent, the smaller the stent thickness, the better the stent is apposed to the vascular wall, and the less the shear disturbance of the stent strut on the blood flow inside the blood vessel, which is more helpful for preventing thrombosis formation, and is helpful for shortening the period of time needed for the stent to be completely absorbed by the body. Therefore, with the development of technology, the framework of a metal-based absorbable stent becomes thinner and thinner. When the stent wall thickness is reduced to a certain extent, the visibility of the stent framework deteriorates, leading to the need for disposing a radiopaque structure to improve the entire visibility of the stent.
To address the shortcoming of low visibility of the absorbable stent framework, as disclosed in the prior art, the stent is wound with a wire made of a highly-radiopaque material at one end, alternatively, the stent framework is partially made of a highly-radiopaque material, or the framework is coated with a highly-radiopaque material, to form a radiopaque structure for improving the entire visibility of the stent. Typically, the highly-radiopaque material refers to a metal having a relatively high density, for example, a noble metal. In the prior art, in order to enable the radiopaque structure to have sufficient visibility, the coating thickness or wire diameter needs to be increased, which significantly increases the thickness of the stent, leading to the risk of thrombosis in the relatively thick stents, and increases the crimped diameter of the stent.
The crimped diameter is one of the important parameters characterizing the mechanical property of a stent, which refers to the maximum outer diameter of the stent that is compressed mechanically onto a balloon. After the stent is preloaded on the balloon, the crimped diameter determines the ability of the stent to pass through a stenosis blood vessel of a diseased area. The smaller the crimped diameter, the narrower the blood vessel that the stent can pass through. The value of the crimped diameter also affects the flare effect of the stent, that is, the greater the crimped diameter, the higher the possibility that the proximal and distal ends of the stent may be circumferentially separated from the balloon, and the higher the possibility that the stent may scratch the vascular wall, or deform or peel off from the balloon, when passing through the tortuous site.
Also as disclosed in the prior art, a hole is disposed on the polymer stent framework, and a highly-radiopaque material is pressed into the hole to form a radiopaque structure by means of gluing or welding. Such polymer stents are typically thick, with the thickness being 120 μm or more. Therefore, a relative small area of the radiopaque structure could make a clear image on the DAS equipment, and lead to high visibility accordingly. However, this method of improving the visibility of polymer stents is unsuitable for relatively thin absorbable stents, i.e., stents having the thickness less than 120 μm. This is because when the stent wall is relatively thin, the depth of the hole is small, and the thickness of the radiopaque material compressed in the hole is smaller. In the case that the area of the radiopaque structure is small, if the thickness of the radiopaque structure is relatively small, the image formed under the DSA equipment is unclear and hard to be identified. In addition, in the case that the stent thickness is constant, if the area of the radiopaque structure described above is increased only by increasing the area of the hole on the stent framework, the space of the stent framework occupied by the radiopaque structure may be increased, which may affect the deformation of the stent when the stent is crimped, thereby affecting the crimped diameter of the stent. Therefore, a suitable radiopaque structure should be disposed on an absorbable stent with thickness less than 120 μm to meet the visibility requirements, while not having a substantial affect on the crimped diameter of the stent.